Implantable cardiac devices (“ICDs”) are well known in the art, and may operate to treat a variety of heart conditions. Generally ICDs are designed to monitor and stimulate the heart of a patient who suffers from a cardiac arrhythmia. Using leads connected to a patient's heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart's regular rhythm.
ICDs may treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the ICD. The program, which is responsible for the operation of the ICD, may be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Programmable ICDs are generally of two types: single-chamber ICDs, and dual-chamber ICDs. In a single-chamber ICD, stimulation pulses are provided to, and cardiac activity is sensed within, a single-chamber of the heart, either the right ventricle or the right atrium. In a dual-chamber ICD, stimulation pulses are provided to, and cardiac activity is sensed within, two chambers of the heart, namely both the right atrium and the right ventricle. The left atrium and left ventricle may also be sensed and paced, provided that suitable electrical contacts are effected therewith.
In general, both single and dual-chamber ICDs are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., the chamber where a stimulation pulse is delivered) with a “V” indicating the ventricle, and “A” indicating the atrium, and a “D” indicating both the atrium and the ventricle. The second letter of the code identifies the chamber where cardiac activity is sensed, using the same letters to identify the atrium, ventricle, or both, and where an “O” indicates that no sensing takes place.
The third letter of the code identifies the action or response taken by the ICD. In general, three types of actions or responses are recognized: (1) an Inhibiting (“I”) response, in which a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response, in which a stimulation pulse is delivered to the designated chamber of the heart a prescribed period after a sensed event; or (3) a Dual (“D”) response, in which both the Inhibiting response and Trigger response are invoked, inhibiting in one chamber of the heart and triggering in the other.
A fourth letter, “R”, is sometimes added to the code to signify that a particular mode identified by the three letter code is rate-responsive, in which the pacing rate may be adjusted automatically by the ICD, based on one or more physiological factors, such as blood oxygen level or the patient's activity level. As used herein, “(R)”, for example, DDD(R), refers to the occurrence of two modes, such as DDD and/or DDDR.
Thus, for example, a DDI pacemaker is capable of sensing and pacing in both chambers, and operates in a non-atrial tracking mode, i.e., it inhibits ventricular stimulation pulses when a prior ventricular activity is sensed.
A DDDR ICD represents a fully automatic ICD that is capable of sensing and pacing in both the atrium and ventricle, and is also capable of adjusting the pacing rate based on one or more physiological factors, such as the patient's activity level. In general, DDD(R) pacing has four functional states: (1) P-wave sensing, ventricular pacing; (2) atrial pacing, ventricular pacing; (3) P-wave sensing, R-sensing; and (4) atrial pacing, R-wave sensing. Advantageously, for the patient with complete or partial heart block, the DDD(R) ICD tracks the atrial rate which is set by the heart's SA node, and then paces in the ventricle at a rate that follows this atrial rate. Because the rate set by the SA node represents the rate at which the heart should beat in order to meet the physiological demands of the body (at least for a heart having a properly functioning SA node), the rate maintained in the ventricle by such an ICD is truly physiologic.
One problem facing the advent of dual-chamber ICDs is that when an ICD delivers a stimulation pulse to the ventricle during an appropriate portion of a cardiac cycle, this pulse may be sensed by the atrial channel. Therefore, it is common practice in the art to apply a post-ventricular atrial blanking (“PVAB”) period upon delivery of a ventricular stimulation pulse in order to prevent the saturation of the sense amplifiers of the atrial channel. During a PVAB period all cardiac activity is ignored by the atrial channel. Because ventricular and atrial pulses are sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an “after potential,” formed at the electrodes, may corrupt the evoked response, which is sensed by the sensing circuits. This undesirable situation occurs frequently because the polarization signal may be three or more orders of magnitude greater than the evoked response.
Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time. By disabling the atrial sense amplifier, that is, applying a refractory or “blanking” period upon the delivery of a ventricular stimulating pulse, the atrial sense amplifier is not effected by the ventricular stimulation pulse. At a specified time interval after the delivery of a ventricular stimulating pulse, the atrial sense amplifiers are enabled again to sense intrinsic or evoked atrial events.
However, the PVAB period poses a problem in that it may occur mid-way or even late in the atrial cycle and may therefore result in an inability of the atrial channel to sense the next intrinsic atrial event. Essentially, the atrial channel is “blinded” to rapid atrial rates precluding proper diagnostic and therapeutic measures by the ICD. For example, a missed atrial event could trigger an atrial stimulation pulse to be inappropriately delivered by the ICD or, in ICDs programmed to operate in one of a plurality of operating modes, cause the ICD to inappropriately switch modes. Such inappropriate pacing or sensing could endanger the patient by inducing a sequence of events that might induce cardiac arrhythmias.
Another problem facing the development of dual-chamber ICDs is that the evoked R-wave (the electrical signal associated with ventricular contraction) subsequent to a ventricular stimulation pulse will typically propagate to the atrium in patients with intact atrioventricular (“AV”) conduction. This propagated signal of a ventricular R-wave in the atria is commonly referred to as “far-field R-wave” (FFR). Even a premature ventricular contraction (“PVC”), an arrhythmic event common in many patients who require implantable cardiac devices, may propagate and produce a far-field signal on the atrial channel. Such far-field signals sensed by the atrial channel could be interpreted as atrial events. This erroneous sensing could be misinterpreted by the controlling operations of the ICD as a change in atrial rate or even an atrial arrhythmia and consequently invoke improper therapeutic measures, potentially harming the patient.
In order to overcome this risk, the PVAB period employed upon the delivery of a ventricular pulse is commonly programmed long enough to encompass the far-field signal associated with the propagation of a ventricular R-wave subsequent to a ventricular stimulation pulse. This PVAB period is commonly programmed to be a fixed time interval, typically 150 msec. However, this relatively long, fixed PVAB period may exacerbate the limitations of a dual-chamber device in that the ability of the ICD to detect rapid atrial rates may be further impaired. Devices use a rate branch algorithm for SVT discrimination. If the atrial signal is not sensed appropriately, the device could miss diagnose the arrhythmia as VT in stead of SVT and deliver inappropriate therapy.
The window of time that the atrial channel is enabled for sensing atrial events is directly shortened as the PVAB period is lengthened to eliminate far-field signals from being sensed. Furthermore, conduction time between the ventricle and the atrium will vary from patient to patient. In some patients, far-field signals associated with ventricular events may occur even later than the typically programmed 150 msec blanking period. Using still longer blanking periods could more severely impair the ability of the ICD to detect even normal atrial rates.
Maximizing the window of time that the atrial channel is enabled for sensing atrial events is particularly important for ICDs that are programmed to operate in one of a plurality of possible operating modes.
ICDs capable of operating in a plurality of modes are important because, for example, a given patient may develop fast atrial rhythms that result from a pathologic arrhythmia, such as a pathological atrial tachycardia, atrial fibrillation, or atrial flutter. In these cases, a DDD(R) ICD may pace the ventricle in response to the sensed atrial arrhythmia up to a programmed maximum tracking rate (“MTR”).
Occasionally it is possible at the time of implantation of an ICD to determine whether an atrial tachycardia, atrial fibrillation, or atrial flutter condition is going to develop. In such instances, the ICD may be programmed to operate in a different mode of operation, the leads may be repositioned within the heart, or other actions may be taken to minimize the likelihood of such pathologic arrhythmias occurring. However, it is not always possible at the time of implantation to determine whether a patient will develop an atrial arrhythmia after the ICD is implanted.
Therefore, if such pathologic arrhythmias subsequently occur, they must be treated using other techniques, such as through the administration of medication, which generally requires the attendance of a physician. However, a physician is not always present when such pathologic arrhythmias develop, and even when a physician is available, such medication also may suppress undesirably the ability of the SA node to increase the sinus rate during periods of exercise, emotional stress, or other physiologic stress. Thus, the use of such medication may prevent the ICD from functioning as an intrinsic physiological rate-responsive pacemaker.
As a result, attempts have been made in the art to prevent undesirable tracking of pathologic atrial arrhythmias by automatically switching the mode of operation of the ICD from an atrial tracking pacing mode to a non-atrial tracking pacing mode. For example, an atrium-controlled ICD has been described, wherein the ICD temporarily switches from an atrial tracking mode to a non-atrial tracking mode for a fixed number of stimulation pulses if the sensed atrial activity indicates an atrial arrhythmia may be developing.
Additionally, an atrial tracking ICD with automatic mode switching capability has been disclosed. This ICD has the capability of setting a tachycardia rate limit (“TRL”) or tachycardia detection rate (“TDR”) slightly above an MTR, so that mode switching to a non-atrial tracking mode occurs when the TRL or TDR is exceeded. A third threshold rate is also set at a value below the MTR. The ICD switches back to an atrial tracking mode when the patient's atrial rate drops below this third threshold. To avoid mode switching based on a single short atrial interval between atrial events, the atrial rate is continuously averaged over several cycles. This technique effectively prevents frequent mode switches in patients whose atrial rates “hover” around the MTR.
Also described is an implantable dual-chamber ICD programmed to operate primarily in an atrial tracking mode. This ICD automatically switches its mode of operation from the atrial tracking mode to a non-atrial tracking mode in the event a filtered atrial rate exceeds a prescribed upper rate limit. This mode switching is accompanied by a corresponding switching from a primary set of operational parameter settings for the primary mode, to an alternate set of operational parameters for the alternate mode.
One parameter shared by mode-switching algorithms that cause the ICD to switch from atrial tracking mode to a non-atrial tracking mode is a step that monitors events sensed by the atrial channel to determine whether a pathologic arrhythmia has occurred such that the mode-switching algorithm should be invoked. Thus it would be desirable to provide a system and method for automatically adjusting the post-ventricular atrial blanking period such that the blanking period following a ventricular stimulation pulse is minimized, thereby allowing the longest atrial sensing window possible in a mode-switching ICD. Furthermore, it would be desirable to implement the system and method in a way that allows far-field signals sensed by the atrial channel to be properly interpreted as the ventricular events that they are associated with, thereby excluding them from atrial rate determinations. It would further be desirable to enable the ICD to perform this automatic PVAB period adjustment without requiring dedicated circuitry and/or special sensors.